1. Field of the Invention
The present invention relates to disk drives for computer systems. More particularly, the present invention relates to a disk drive comprising oscillators and counters for sensing current in a voice coil motor.
2. Description of the Prior Art
FIG. 1 shows a prior art disk drive comprising a disk 2 rotated about a center axis by a spindle motor (not shown). A head 4 attached to a distal end of an actuator arm 6 is actuated radially over the disk 2 by a voice coil motor (VCM) 8. The VCM 8 comprises a voice coil 10 which interacts with permanent magnets of a VCM yoke in order to rotate the actuator arm 6 about a pivot. The VCM 8 is typically driven in either a linear mode or in a pulse width modulated (PWM) mode. In addition, the motion of the VCM 8 may be controlled using a current feedback loop by sensing the amount of current flowing through the voice coil 10 which is proportional to the amount of torque applied to the actuator arm 6.
FIG. 1 also shows a VCM driver 12 comprising a conventional H-bridge driver for driving the voice coil 10 shown as a resistance Rvcm 14 and an inductance Lvcm 16. The H-bridge driver comprises a plurality of driver switches 18A-18D for selectively connecting the ends of the voice coil 10 to a supply voltage 20 or to ground 22 depending on the desired direction of rotation. A plurality of diodes D1-D4 protect the driver switches 18A-18D from flyback currents generated from driving an inductive load.
In order to control the motion of the VCM 8 accurately using a current feedback loop it is important to measure the total integral of the current flowing through the voice coil 10. Several problems arise when attempting to use the conventional techniques for sensing the current flowing through the voice coil 10 when driven in a PWM mode. Referring again to FIG. 1, if a single sense resistor Rsense 24 in series with the voice coil 10 is used to sense current, the PWM voltage appears on both sides of the resistor Rsense 24 at several volts at very high slew rates. This chop voltage (a square wave) must be rejected by sense amplifier 31 so that the very small voltage across Rsense 24 can be accurately measured. This high frequency AC voltage capacitively couples into the sense amplifier 31, and creates offsets and nonlinearities which distort the current sense measurement. This problem exacerbates as the frequency of the PWM increases.
Another prior art current sensing technique uses a sense resistor Rsensep 26 in series with the supply voltage 20 and an amplifier 28, or a sense resistor Rsensem 30 in series with ground 22 and an amplifier 32. This technique avoids the common mode voltage problem associated with sense resistor Rseries 24, however, it also leads to other problems related to inductive flyback currents. Assume, for example, that current is flowing to the right through the voice coil 10. Initially, driver switches 18A and 18D are on, allowing Vpwr 20 to source the current through the sense resistors Rsensep 26 or Rsensem 30. Driver switch 18A is driven by a PWM signal, while driver switch 18D is left on continually. When the PWM signal turns driver switch 18A off, the inductive load keeps current flowing to the right in the coil regardless of the voltage applied because of the magnetic flux stored in the coil and its magnetic structure. This inductive current can cause diode D2 or driver switch 18B to conduct current, depending on the ratio of impedances. Since current is also flowing through switch 18D, the flyback current momentarily cancels the current through sense resistor Rsensem 30, which can distort the current sense measurement by creating a blank spot in the voltage waveform. Additionally, if the two halves of the H-bridge are switched alternately, flyback current from the inductive current can drive the voltage at the top of sense resistor Rsensem 30 below ground. When this happens, substrate parasitic transistors (shown as parasitic transistor 31 in FIG. 1) are activated, drawing current from elsewhere in the driver circuitry in a random manner, both distorting the current measurement with this additional current and disrupting operation of the driver circuitry.
Regardless of how the H-bridge PWM switching is timed, shootthrough currents (caused by a brief simultaneous conduction between driver switches 18A and 18B or driver switches 18C and 18D) or gate charge injections can also create false values for current that distort the true measurement. These problems are generally avoided using sample/hold circuits 34 and 36, which sample the voltage across the resistors 26 and 30 at a point in the PWM chop cycle where distortions due to flyback, shootthrough, switching, or diode conduction, do not occur. However, the sampling process adds delay to the loop. Additionally, an abrupt change from a large current to a small current leaves a time related sample distortion in the waveform that can be larger than the actual voltage value corresponding to the small current. The control system spends time responding to these spurious distortions which cause unwanted motion in the VCM. Still further, the sense amplifiers 28 and 32 must be designed such that their inputs can be driven below ground, or above the power supply, respectively, in order to sense current of all polarities. Sensing current above or below the power supply rails significantly increases the circuit complexity of a monolithic IC sense amplifier due to substrate current injection, which also removes current from the sense resistor in a nonlinear manner.
There is, therefore, a need to accurately sense the current flowing through the voice coil of disk drive VCM in order to implement a current feedback loop while driving the VCM in a PWM mode.